Those who believe in the "Year-Day Theory" try to make Scriptures imply that one day equals one thousand years. They use 2 Peter 3:8 to support their theory which says "one day with the Lord is like a thousand years."
Date setters often use the Year-Day theory to try to predict the day of the Rapture and the Second Coming of Christ by substituting "one thousand years" into passages that use "one day." Some people teach that because the earth was created in 7 days, this equals 7,000 years of human history. They add the 4000 years in the Old Testament plus 2000 years in the New Testament to correspond to the first 6 days. Using this formula, they believed Jesus would return in the year 2000 and set up His Millennial Kingdom for 1,000 years, which would fulfill the 7th day. Obviously, they were wrong, so you don't hear much about this theory anymore.

Others have tried to plug the Year-Day Theory into Daniel 8:13-14, which says, "How long will the vision about the regular sacrifice apply, while the transgression causes horror, so as to allow both the holy place and the host to be trampled?" He said to me, "For 2,300 evenings and mornings; then the holy place will be properly restored." These people substituted years for days in this passage, coming up with 2,300 years, which they tried to use to predict the Second Coming.

However Daniel 8:14 says, "2,300 evenings and mornings" which clearly means literal days not years. William Miller plugged the Year-Day theory into this passage to wrongly predict the Second Coming of Christ on October 22, 1844, known as "The Great Disappointment."
If we read the entire verse, 2 Peter 3:8 says, "But do no let this one fact escape your notice, beloved, that with the Lord one day is LIKE a thousand years, and a thousand years LIKE one day." This passage teaches that God exists outside of the realm of time, not that a thousand years are equal to one day. Don't fall into the trap of believing the Year-Day theory.